Copper electroplating in a citric acid bath

ABSTRACT

A NEW METHOD FOR ELECTROPLATING COPPER ON AN ARTICLE UTILIZING AQUEOUS COPPER SALT-CITRIC ACID SOLUTION IN WHICH CITRIC ACID IS THE PRINCIPAL COMPLEXING AGENT FOR THE COPPER ION IN THE SOLUTION. CUPRIC SALTS ARE PREFERRED IN THIS INVENTION, ESPECIALLY CUCO3-CU(OH)2 AND CUSO4-5H2O, FOR BOTH PLATING SOLUTIONS AND STRIKE SOLUTIONS. FOR PLATING THE PH RANGE IS ABOUT 0.4 TO 8.9, THE CUPRIC ION CONCENTRATION IS ABOUT 16 TO 64 GRAMS PER LITER, AND THE CITRIC ACID CONCENTRATION IS ABOUT 48 TO 192 GRAMS PER LITER. FOR STRIKE SOLUTION THE PH RANGE IS ABOUT 1 TO 12, THE CUPRIC ION CONCENTRATION IS ABOUT 3 TO 16 GRAMS PER LITER, AND THE CITRIC ACID CONCENTRATION IS ABOUT 20 TO 100 GRAMS PER LITER. THE CURRENT DENSITY IS ABOUT 1 TO 40 AMPERES PER SQUARE FOOT FOR BOTH PLATING AND STRIKE SOLUTIONS. THIS NEW METHOD AVOIDS THE HANDLING PROBLEMS OF USING CYANIDE OR MINERAL ACIDS AS THE COPPER COMPLEXING AGENT AND GIVES COMPARABLE PLATING RESULTS.

United States Patent U.S. Cl. 204-40 2 Claims ABSTRACT OF THE DISCLOSUREA new method for electroplating copper on an article utilizing aqueouscopper salt-citric acid solutions in which citric acid is the principalcomplexing agent for the copper ion in the solution. Cupric salts arepreferred in this invention, especially CuCO -Cu(OH) and CuSO -5H O, forboth plating solutions and strike solutions. For plating the pH range isabout 0.4 to 8.9, the cupric ion concentration is about 16 to 64 gramsper liter, and the citric acid concentration is about 48 to 192 gramsper liter. For strike solutions the pH range is about 1 to 12, thecupric ion concentration is about 3 to 16 grams per liter, and thecitric acid concentration is about 20 to 100 grams per liter. Thecurrent density is about 1 to 40 amperes per square foot for bothplating and strike solutions. This new method avoids the handlingproblems of using cyanide or mineral acids as the copper complexingagent and gives comparable plating results.

BACKGROUND OF THE INVENTION Metal finishing companies are currentlygiving serious consideration to treatment of their waste solutions. Inparticular, most electroplating shops utilize copper cyanide platingbaths, for which disposal and break down of the cyanide involvesparticularly rigorous and expensive treatment methods due to theextremely poisonous nature of cyanide.

Furthermore, when copper is electroplated on a more electronegativemetal such as steel or zinc, there is a tendency for the copper to beinitially deposited by displacing some of the metal at its surface. Thisgalvanic-type deposit so obtained is nearly always loose andnon-adherent, preventing the subsequent electrodeposition of a goodcopper coating. This necessitates imparting an initial strikecoating ofcopper from a separate copper bath having a very low copper-ionconcentration. The copper electrodeposit is then applied from anordinary plating bath having much higher copper-ion concentrations.

Although copper baths with cyanide as the major complexing agent for thecopper ion provide the best strike and ordinary copper coating, it wouldbe highly desirable to develop the use of equally eilective non-cyanidebaths due to the inherent disadvantages of using cyanide-based baths.

Non-cyanide copper plating is not new. Formulations based on coppersulfate and copper fluoborate are used in acid copper plating, andpyrophosphate baths are used commercially for alkaline copper plating.These noncyanide baths, however, do not give equivalent performance tothose containing cyanide. Copper sulfate-sulfuric acid solutions cannotbe used as strike solutions on steel due to galvanic action whichresults in a non-adherent, galvanic-type deposit. Furthermore, on zincand zinc alloys these acid solutions rapidly attack the zinc surface andchange the dimensions of the article being plated. Both the fluoborateand pyrophosphate baths require initial cyanide bath copper-strikecoatings on steel and zinc diecasts in order to get a consistently goodordinary 3,684,666 Patented Aug. 15, 1972 copper coating from both ofthese baths. Consequently, although the use of cyanide may be reduced,it has not been eliminated.

Prior art methods of electrodepositing copper on a metallic substratehave utilized citric acid, or a salt thereof, as an additive to copperelectroplating solutions to increase the permissible cathode currentdensity range; to increase the conductivity of the bath; to increase thebrightness of the cathode deposit; to increase the tolerance of the bathto impurities; and to aid in anode cor IOSlOIl.

Although often heretofore used as an additive, citric acid or its saltshave never beiore been used as the grirflcipal complexing agent forcopper in electroplating at s.

SUMMARY OF THE INVENTION In contrast to the prior art, it has beensurprisingly found that copper can be electrodeposited on an articlewhich is made the cathode by exposing the article to an aqueous cupricsalt solution at a pH from about 0.4 to 8.9, the solution containingfrom about 16 to 64 grams per liter of cupric ion and from about 48 to192 grams per liter of citric acid, and applying a current density ofabout 1 to 40 amperes per square foot to obtain the desired coating.

Where the article to be copper-coated consists of a metal that is moreelectromegative than copper, and which therefore will almost alwaysresult in non-adherent ga1vanic-type deposits of copper on the article,an initial strike-coating of copper is imparted on the article prior toelectrodepositing the ordinary copper coating on the article. It isagain surprisingly found that the copper strike-coating can be impartedon the article by exposing the article to an aqueous cupric saltsolution at a pH from about 1 to 12, the solution containing from about3 to 16 grams per liter of cupric ion and from about 20' to grams perliter of citric acid, and applying a current density from about 1 to 40amperes per square foot to obtain the desired strike-coating.

Thus, citric acid can be used as the principal complexing agent forcopper in strike and plating solutions, giving comparable results tocyanide and acid plating baths but avoiding the disadvantages of theiruse.

DETAILED DESCRIPTION OF THE INVENTION By using the present invention onemajor advantage is the elimination of cyanide and pyrophosphate ions inthe electroplating bath and in the subsequent rinse water. Thus, wastedisposal is made relatively easy in that the citrates are readilybiodegradable.

A further advantage is that the copper deposit has inherent brightnessas compared to similar deposits obtained from conventionalnon-proprietary copper electroplating solutions.

Still another advantage of the present invention is the elimination ofthe need for a cyanide strike bath. Other copper electroplating baths,such as those based on fluoborate or pyrophosphate, require :a cyanidestrike before adequate adhesion of the copper plating can be obtained.By this invention it is possible to adjust the quantities of citrate andcopper salt to perform both the striking and electroplating.

Furthermore the effect of storing citric acid-based electroplatingsolutions is insignificant with respect to the copper electrodepositssubsequently obtained using such solutions.

According to the present invention, bright, adherent, and dense coatingsof copper can be electrodeposited on an article by exposing the articleas the cathode to an aqueous cupric salt solution in which citric acidis the principal complexing agent for the copper in solution. Ordinarilythe anode that is contemplated to be used in carrying out the steps ofthe invention is copper metal.

The article to be electroplated may be composed of any suitable metalsuch as aluminum, steel, zinc, brass, or other metals and their alloys.Where the article is composed of a metal that is more electronegativethan copper, galvanic-type deposits will result on the article surfacewithout the application of any current density. Immersion-type depositsare usually not of good quality and are non-adherent, resulting in apoor quality copper coating when the current density is subsequentlyapplied. This necessitates imparting to the article an initial coppercoating, known as a strike-coating, to provide an initial adherentcoating and to prevent the formation of an undesirable galvanic-typecoating. Thus, it is necessary to impart a strike-coating of copperwhere the article to be plated is composed of steel or zinc, which aremore electronegative than copper; whereas it is not necessary to imparta strike-coating to an article composed of brass. However, the coppergalvanic-type coatings encountered where citric acid is the principalcomplexing agent for the copper in solution, have been found to besurprisingly more adherent than those normally encountered where othercomplexing agents are used. This copper coating probably occurs due togalvanic displacement by the copper and may even be due to some chemicalreduction.

Although any water-soluble cupric or cuprous salt can be used inpracticing this invention, it is preferred to use cupric salts andespecially preferred to use cupric sulfate (e.g., CuSO and CuSO -H O) orcupric carbonate (e.g., 2CuCO -Cu(OH) and CuCO -Cu(OH) due to theirready availability and price.

The temperature of the plating baths may vary from room temperature tothe boiling point of the solution, although for convenience thetemperature is kept at room temperature (about 2030 C.).

The optimum current density is dependent on other electrolysisconditions and the composition of the bath. Current densities of about 1to 40 amperes per square foot have been found satisfactory.

The quantity and quality of the copper coatings attained are stronglydependent upon the relative concentrations of cupric ion and citricacid, as well as the pH of the plating solution. It is preferred to usecitric acid in practicing the invention, although the use of citrates isnot excluded thereby. The preferred concentration of citric acid inordinary copper plating baths is from about 48 to 192 grams per liter,and it is most preferred to use 160 to 192 grams per liter. In copperstrike-coating baths it is preferred to have a concentration of citricacid from about 20 to 100 grams per liter.

A cupric ion concentration of about 16 to 64 grams per liter ispreferred for ordinary plating baths, and a concentration of 50 to 64grams per liter is most preferred. For strike-coating baths a cupric ionconcentration of about 3 to 16 grams per liter is preferred. It isespecially preferred to have a cupric ion concentration of about 12 to16 grams per liter when cupric carbonate is used, and a concentration ofabout 3 to 5 grams per liter when cupric sulfate is used.

The preferred pH with respect to the quality of the copper depositedwill vary somewhat with the concentrations of the bath make-upingredients. The pH for ordinary copper plating baths can be from about0.4 to 8.9, with a pH from about 0.4 to 1.2 preferred when cupricsulfate is used and a pH from about 7 to 8 preferred when cupriccarbonate is used. For copper strike-coating baths the pH can be fromabout 1 to 12, and a pH from about 1 to 2 is preferred when cupricsulfate is used and a pH from about 9 to 12 is preferred when cupriccarbonate is used.

Although not essential to the use of the invention, sodium sulfate orammonium citrate can be added to improve the conductivity of the platingbath and boric acid can be added as a buffering agent, as well as a base(e.g., NH OH, NaOH, etc.) for pH control. Of course, this does notexclude the use of other additives to improve the conductivity of thebath or to control the bath pH, which can be determined by someoneskilled in the art.

A preferred embodiment of the invention involves using an aqueous copperstrike solution containing about 96 grams per liter of citric acid, 20grams per liter of CuCQ -Cu(OH) adjusting the pH to about 11.6 byaddition of NaOH, maintaining it at a current density of 20 to 40amperes per square foot, and a plating time of 5 minutes.

Another preferred embodiment of the invention involves using an aqueouscopper strike solution at a pH of bout 1.6 containing about 24 grams perliter of citric acid, 16 grams per liter of CuSO -5H O, 6 grams perliter of sodium sulfate, 20 grams per liter of boric acid, maintainingthe current density at 1-3 amperes per square foot, and a plating timeof 5 minutes.

Still another preferred embodiment of the invention involves using anaqueous copper plating solution at a pH of about 0.8 containing about192 grams per liter of citric acid, 250 grams per liter of CuSO -5H O,maintaining the current density at 20 to 40 amperes per square foot, anda plating time of 60 minutes.

A final preferred embodiment of the invention involves using an aqueouscopper plating solution containing about 192 grams per liter of citricacid, grams per liter of CuCO -Cu(OH) adjusting the pH to about 8 byaddition of NH OH, maintaining it at a current density of 20 to 40amperes per square foot, and a plating time of minutes.

The following examples are provided for illustrative purposes and shouldnot be interpreted as limiting the invention, the scope of which isdefined by the appended claims.

EXAMPLE I A standard Hull Test Cell is used as the electrolyzingapparatus. The cathode consists of a standard steel panel which is 4"long, 2 /2" wide, and 0.010" thick. A pure copper anode is utilizedalso.

The cathode is cleaned and prepared according to standard electroplatingprocedures consisting of degreasing, alkaline cleaning and thoroughwater rinsing before being electroplated.

The cathode is then exposed to an aqueous copper strike solution at a pHof 1.6 containing 24 grams per liter of citric acid, 16 grams per literof CuSO -5H O, 6 grams per liter of sodium sulfate, and 20 grams perliter of boric acid. The solution is maintained at room temperature,while the current is I to 3 amperes. After 5 minutes immersion time thecathode is withdrawn from the strike solution and thoroughly rinsed withwater, resulting in a bright, thin, adherent copper strike coating.

EXAMPLE II The copper-strike coated steel panel of Example I is thenexposed as the cathode to an aqueous copper plating solution containing95 grams per liter of 192 grams per liter of citric acid, and the pH isadjusted to 8 by addition of NH OH. The solution is maintained at roomtemperature, while the current density is 20-40 amperes per square foot.After an immersion time of 120 minutes the steel panel is withdrawn fromthe plating solution resulting in a bright, dense, and adherent coppercoating.

EXAMPLE III A standard Hull Test Cell brass panel is cleaned andprepared according to standard electroplating procedures and is exposedas the cathode to an aqueous copper plating solution at a pH of 0.4containing 250 grams per liter of CuSO -5H O and 192 grams per liter ofcitric acid, a

temperature of 150 F., and a current of 1 ampere. A good copper platingis attained on the brass panel after an immersion time of 3 minutes, inthe Hull Test Cell. Similar results are obtained at a current density of20-40 amperes per square foot and a pH of 0.8, in larger baths, forlonger immersion times. t.

EXAMPLE IV Satisfactory results are obtained following the procedure ofExample III except that the plating bath at a pH of 1.2 contains 63grams per liter of CuSO -5H O and 48 grams per liter of citric acid, andis maintained at a current density of 40 amperes per square foot, andthe immersion time is 60 minutes.

' EXAMPLE v Satisfactory results are obtained following the procedure ofExample III except that the plating bath at a pH of 8.9 contains 250grams per liter of CuSO -5H O and 192 grams per liter of citric acid,and is maintained at a current density of 20 amperes per square foot,and the immersion time is 120' minutes.

EXAMPLE VI Similar results are obtained following the procedure ofExample I except that the strike solution at a pH of 9.2 contains '29grams per liter of CuCO -Cu(OH) 86 grams per liter of citric acid, and61 grams per liter of ammonium citrate, and is maintained at a currentdensity of 40 amperes per square foot, and the immersion time is 60minutes, ina larger bath.

Thickness tests were determined by means of a Kocour ElectronicThickness Tester Model 955, utilizing the appropriate anodic strippingsolution. The tester was first balanced and the sensitivity checked. Itwas then calibrated against standard panels for copper deposit thicknesson steel substrates. Thickness tests were performed on the Hull Cellpanels at locations which were indicative of current densities of 4 ampsper sq. ft. and amps per sq. ft.

The throwing power measurement is obtained by taking deposit thicknessreading at 40 and 4 amps per sq. ft. The relationship normally expectedwould be a deposit ratio of 10 to 1. The calculation for throwing powerin terms of the primary and metal ratios is as follows:

Primary ratiometal ratio Primary ratio The primary ratio is based on thecurrent densities at which the two thicknesses are measured, in ourcase, 10.

The metal ratio is based on the thickness actually obtained at thosecurrent densities. For example, in Table I,

X 100 percent Panel Tl gave a thickness ratio at the two current densi-TABLE I.-THROWING POWER TESTS Plated thickness Solution pH of copper atPercentage im- Solution composi- Plat- 40 a.s.f. 4 a.s.t. prove- MetalPanel No. tion Strike ing (inch) (inch) ment ratio T1 CNST and CNRO- 11.88 10. 28 0. 00019 0. 00007 73 2. 7 T2. CNST and acid..- 11. 88 0. 48 0.00021 0. 00005 58 4. 2 T3- V and A 1. 69 0. 75 0. 00022 0. 00005 56 4. 4T4 H and Q ll. 68 7.73 0.00015 0 00003 50 5. 0

Solution Composition CNST=0.2M copper cyanide, 0.7M sodium cyanide,

0.05 sodium hydroxide, pH 11.9;

CNRO =0.2M copper cyanide, 0.7M sodium cyanide, 0.3M sodium carbonate,0.16 Rochelle salt, pH 10.3; V

=0.064M copper sulfate, 0125M citric acid, 0.042M sodium sulfate, 0.32M

horie acid, pH 1.7; ACID-=0.8M copper sulfate, 0.5M suliuric acid, pH0.5; A==1.0M citric acid, 1.0M cop per sulfate, pH 0.8; H=0.5M citricacid, 0.09M copper carbonate, pH 11.6

adjusted by addition of sodium hydroxide; Q=1.0M citric acid, 0.43Mcopper carbonate, pH 7.8 ad usted by addition of sodium hydroxide.

N orELCopper sulfate means CuSOr-fiHaO; a.s.t. means amperes per squarefoot.

EXAMPLE v11 Similar results are obtained following the procedure of ofExample I except that the strike solution contains 20 grams per liter ofCuC0 -Cu(0H) and 96 grams per liter of citric acid, the pH beingadjusted to 12 by additionof NaOH, and maintained at a current densityof 20-40 amperes per square foot, and the immersion time is 1 minute, ina larger bath.

EXAMPLE VIII Throwing power tests Test panels consisted of Steel HullCell panels. According to standard electroplating procedure, four panelswere plated, two from conventional baths and two from copper citrateplating solutions. The conventional bath panels were struck fromsolution composition CNST, and plated from either the coppercyanide-Rochelle salt solution (CNRO) or the acid copper sulfatesolution (ACID). Panels T3 and T4 were struck in the (V) and (H)solutions, and plated in the (A) and (Q) solutions, respectively. Thedetails of the solution conditions can be found in Table I. The strikeplating was performed in two-liter beakers at three volts for one minuteat room temperature. The plating was conducted in Hull Cells at twoamperes and room temperature for five minutes.

Copper carbonate means-OuCOr Cu(OH)| EXAMPLE IX Thickness and adhesiontests Additional panels were prepared by electrolyzing conventional andexperimental strike: and plating solutions using the techniquesdescribed above. These were standard steel Hull Cell panels 0.010 inchthick which were fabricated into bent cathodes to obtain varying currentdensities along length of immersed cathode. This configuration createshigh and low current density areas for evaluation of throwing power,which is the ability of the solution to deposit a relatively uniformthickness over a surface.

The panels rwere cleaned by standard methods and were struck insolutions CNST, (V), and (H), at three to five ivolts at roomtemperature for one minute. They were then electroplated using solutioncompositions CNRO', ACID, (A), and (Q) at 20 to 40 amps per sq. ft. atroom temperature for thirty minutes. The experimental details are shownin Table H.

The plated panels were then bent at angle approximately one-half inchfrom the end opposite the bent cathode end, and the bend was observedfor lifting of the deposit. The panel was then further bent 9-0 in thesame direction, and then bent back to its original flat position.

The bent cathode portion of these same panels were then flattened andthickness tests were performed using the Kocour Tester previouslyreferred to in Example VIII. Five thickness measurement locations wereselected for their range of high to low current density areas. Thesethickness locations are indicated in Table II as No. 1, No. 2, No. 3,No. 4 and No. 5.

8 rent density from about to 40 amperes per square foot until thedesired copper strikeecoating has been deposited on said article, and(2) then electrodepositing a second coating of copper on the resultingarticle which comprises exposing said thin-coated article as the cathodeto an aqueous cupric salt solution at a pH from about 0.4 to 8.9, saidsolution comprising from about 16 to 64 grams TABLE II.THICKNESS ANDADHESION TESTS Average Thickness (inch) Plating Solution rate Panel No.Composition Deposit adhesion No 1 No. 2 No. 3 No. 4 No. 5 (in./hr.2)

B1 CNST and aeid Good 0. 00130 0.00178 0.00058 0.00109 0. 00224 0.00282CNSI and CNR do 0. 00181 0.00172 0.00106 0.00127 0. 00286 Q, Lifted 3dbend 0 00028 0. 00041 0. 00018 0.00017 0.00039 0.00058 CNSI and CN RO do0.00111 0.00127 0. 00094 0.00123 0.00222 -A Lifted all bends 0 00055 0.00008 0.00028 0.00032 0.00082 0. 00106 Good 0. 00041 0. 00024 0.000250.00056 0.00071 See Table I for solution composition.

tion composition (Q) was about one-quarter that of conventional platingIbaths.

Adhesion tests of the deposits plated from copper citrate solutions werenot as satisfactory as those obtained from conventional copper platingsolutions. Of the ten tests made, only two passed the three bends. Thesewere plated from solution composition (Q). All of the panels plated fromsolution composition (A) failed at some point. A

contributing factor in these results is that the deposits from solutioncomposition (Q) are thinner than those from solution composition (A).

EXAMPLE X An adherent galvanic copper coating is obtained on a steelsubstrate utilizing an aqueous solution consisting of 0.064 M coppersulfate, 0.24 M citric acid, 0.042 M sodi urn sulfate, and 0.32 M boricacid. The coating develops at a pH of 1.6 and a temperature of 72 F. inthree to fiive seconds. Adhesion of the deposit is determined by rubbingwith a finger and with a rubber eraser. Galvanic deposition which is notadherent is observed in similarly constituted solutions containingoxalate or citrate instead of citric acid.

What is claimed is:

1. A method of electroplating copper on an article that exhibits greaterelectronegativity than copper in an electroplating bath which comprises(1) electrodepositing on said article a thin copper strike-coating whichcomprises exposing said article as the cathode to an aqueous cupric saltstrike solution of CuCO' -Cu(OH) adjusted to a pH per liter of cupricion and from about 48 to 122 grams per literof citric acid, and applyinga current density from about 1 to 40arnperes per square foot until the'desired copper second coating has been deposited on said article.

2. A method of electroplating copper on an article that exhibits greaterelectronegativity than copper in an elec troplating bath which comprises(1.) electrodepositingon said article a thin copper strike coating whichcomprises exposing said article as the cathode to an aqueous cupricsaltstri ke solution of CuSO -5H O at a pH of about 1.6 containing about6 grams per liter of sodium sulfate and 20 grams per liter of boricacid, said strike solution comprising about 4 grams per liter of cupricion and about 24 gramsper liter of citric acid, and applying a currentdensity from about 1 to 3 amperes per square foot until the desiredcopper strike-coating has been deposited on said article, and 2) thenelectrodepositing a second coating of copper on the resulting articlewhich comprises exposing said thin-coated article as the cathode to anaqueous cupric salt solution at a pH from about 0.4 to 89', saidsolution ion and from about 48 to 192 grams per liter of citric acid,

- and applying a current density from about 1 to 40 amof about 11.6 bythe addition of NaOH, said strike solution comprising about 12 grams perliter of cupric ion and about 96 grams per liter of citric acid, andapplying a curperes per square foot until the desired copper secondcoating has been deposited on said article,

References Cited UNITED STATES PATENTS 2,871,172

